Heretofore, a switching power source apparatus provided with a control circuit is known (see JP2002-233146A). The control circuit causes a voltage to be induced to a secondary winding by switching a switching element provided at a primary side of a transformer between on and off to intermittently apply a voltage to a primary winding, thereby controlling a time when the switching element is turned on in accordance with a state of a load on the secondary side. In this conventional switching power source apparatus, it is disclosed that a period of time when the control circuit is forcibly turned off is provided in a case where a required output value is low, whereby an off state and an on state are repeated to realize electric power saving.
Here, a soft switching system DC-DC converter in which a switching element is turned on or off after a voltage applied to the switching element becomes 0V or a conducting current becomes 0A using a resonance phenomenon is known. The soft switching system DC-DC converter has a characteristic that hard switching is established when a required output value falls below the minimum output value over which soft switching is established. Therefore, a problem occurs that soft switching is not established and a loss thus increases in a case where the technique described in JP2002-233146A is simply applied to the soft switching system DC-DC converter.